optimize listing operation concurrency (#12728)

- remove use of getOnlineDisks() instead rely on fallbackDisks()
  when disk return errors like diskNotFound, unformattedDisk
  use other fallback disks to list from, instead of paying the
  price for checking getOnlineDisks()

- optimize getDiskID() further to avoid large write locks when
  looking formatLastCheck time window

This new change allows for a more relaxed fallback for listing
allowing for more tolerance and also eventually gain more
consistency in results even if using '3' disks by default.

cmd/erasure-common.go

@@ -47,38 +47,6 @@ func (er erasureObjects) getLoadBalancedLocalDisks() (newDisks []StorageAPI) {
 	return newDisks
 }
 
-func (er erasureObjects) getOnlineDisks() (newDisks []StorageAPI) {
-	disks := er.getDisks()
-	var wg sync.WaitGroup
-	var mu sync.Mutex
-	for _, i := range hashOrder(UTCNow().String(), len(disks)) {
-		i := i
-		wg.Add(1)
-		go func() {
-			defer wg.Done()
-			if disks[i-1] == nil {
-				return
-			}
-			di, err := disks[i-1].DiskInfo(context.Background())
-			if err != nil || di.Healing {
-				// - Do not consume disks which are not reachable
-				//   unformatted or simply not accessible for some reason.
-				//
-				// - Do not consume disks which are being healed
-				//
-				// - Future: skip busy disks
-				return
-			}
-
-			mu.Lock()
-			newDisks = append(newDisks, disks[i-1])
-			mu.Unlock()
-		}()
-	}
-	wg.Wait()
-	return newDisks
-}
-
 // getLoadBalancedDisks - fetches load balanced (sufficiently randomized) disk slice.
 // ensures to skip disks if they are not healing and online.
 func (er erasureObjects) getLoadBalancedDisks(optimized bool) []StorageAPI {